Children who are born with a heart containing only one usable pumping chamber (single ventricle) are the leading cause of morbidity and mortality from congenital heart disease (CHD) in the United States. The most common repair is the modified Fontan operation in which the right side of the heart is bypassed and blood is channeled directly to the lungs without a pumping chamber. This surgery is performed in stages. In the initial operation, a connection is created from the aorta to the pulmonary arteries and the outflow from the heart is reconstructed (Stage I). In an intermediate stage, flow from the superior vena cava is channeled directly into the pulmonary arteries while the inferior vena caval flow enters the single ventricle directly. The Fontan completion involves routing inferior vena caval blood to the pulmonary arteries via a baffle or conduit. Patients with single ventricle are also known to have greater neurological problems than the general population and other forms of CHD. One contributing factor is cerebral blood flow (CBF);evidence from preliminary data from our lab as well as in the published literature suggests that the amount of CBF is related to intelligence. There are reasons to believe that CBF in single ventricle patients is different from normal children and changes throughout staged surgical reconstruction. At Stage I, the connection created between the aorta and pulmonary arteries decrease blood pressure and suck blood into the lungs. In the second stage, the brain and lungs are hooked up directly in series with each other and only with each other (blood flows from the aorta to the brain and then directly into the superior vena cava to the lungs). Finally, after complete Fontan reconstruction, brain venous pressures are elevated. The amount of CBF during these stages remain unknown along with the change in CBF with increased carbon dioxide which increases brain flow and is a marker for how well the brain can adapt to meet its blood flow needs. Indeed, there is no data on CBF in normal children in the literature. This is especially important at this age since this rapid growth stage of brain development may be related to ultimate neurological function. Elucidating CBF in single ventricles may ultimately lead to changes in management of these patients;if one stage has an adverse effect on CBF for example and is not normal, it might be prudent to complete the Fontan earlier rather than the typical 18 month timeframe. By accurately measuring CBF which our lab has done previously, magnetic resonance imaging (MRI) offers a unique opportunity to measure CBF non-invasively, CBF response to carbon dioxide as well as brain volumes and abnormalities. Understanding how CBF changes in single ventricles with the stage of surgery as compared to normal children may lead to understanding why they have neurological problems and possibly change medical or surgical management. This is a prospective, non-randomized, single center study built upon preliminary data on CBF in single ventricle patients and other forms of CHD via MRI and seeks to determine CBF changes in single ventricles during staged surgical reconstruction as compared with normal children. Project Narrative: Children who are born with a heart containing only one usable pumping chamber (single ventricle lesion) are the leading cause of morbidity and mortality from congenital heart disease in the United States and suffer from neurological problems. By understanding how blood flow to the brain is different from normals and how it changes during surgery in these patients, we may better be able to understand why they have neurological problems and what can be done to intervene. Changing the way the blood flows in these patients by surgery may alter the way we ultimately manage these patients medically and surgically.